1. Field of the Invention
This invention relates to superconducting electromagnet apparatus. Such apparatus can be used in various applications including nuclear magnetic resonance (NMR) spectroscopy and imaging and Fourier-transform mass spectrometry (FTMS).
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
In conventional superconducting electromagnet apparatus a main superconducting coil assembly of cylindrical form is used to produce a central magnetic field which varies very little over a specified working volume within the bore of the cylinder, that is the so-called homogeneity volume. In many applications of such apparatus, it is necessary for a precise value of the central magnetic field to be set. Furthermore it is often necessary or desirable to make a fine adjustment to this central magnetic field value, for example to reset the central magnetic field to the specified value after it has been altered by use of a passive or superconducting shim coil assembly (which is used to achieve the optimum degree of homogeneity within the homogeneity volume but which can sometimes lead to a slight change in the central magnetic field value). Ideally this fine adjustment should be made without any degradation of the magnetic field homogeneity within the homogeneity volume.
It is known to make use of a so-called B0 shim coil assembly to provide fine adjustment of the central magnetic field value with minimal degradation of the magnetic field homogeneity within the homogeneity volume. Such a B0 shim coil assembly comprises a plurality of superconducting coils connected in series to form a closed loop which is electrically separate from the main superconducting coil assembly but which couples magnetically with the main coil assembly. The B0 shim coils are wound on an outer former which surrounds an inner former on which the main coils are wound and within which the homogeneity volume is located. The B0 shim coil assembly also incorporates a superconducting switch within the loop and current input terminals for adjusting the amount of current passing through the shim coils and for enabling persistence of this current when the required current value is reached and the superconducting switch is closed, the required current value being zero or a positive or negative value. The geometries of the B0 shim coils are chosen so as to have substantially no effect on the homogeneity of the magnetic field within the homogeneity volume. Furthermore the B0 shim coils are constructed using materials such that the maximum required operating current is well below the critical current at which the coils are no longer superconducting and such that there is insignificant drift in the magnetic fields associated with these coils. Thus, in the persisted mode in which the uniform central magnetic field is maintained within the homogeneity volume, the current in the shim coils will remain substantially unchanged ignoring the effect of any inductive interactions with any other part of the electromagnet apparatus or with any external field source. Typically the shim coils are constructed using wire having a critical current of the same order of magnitude as the wire used for the main magnet coils.
In most circumstances the coupling of the B0 shim coil assembly with other shim coil assemblies, which are provided to correct distortions in the magnetic field due to other disturbing influences, is minimal. This can be ensured by appropriate choice of the shim coil geometries. As is well known the superconducting switch of the B0 shim coil assembly can be operated in the same manner as the superconducting switch which is provided for controlling the main coil assembly. In both cases, after the coils have been charged to the required current level from an external current source whilst the loop is open-circuited by opening of the switch, the switch is closed to allow the required constant current level to be maintained within the superconducting loop incorporating the coils. Where the main coil assembly is being initially energised, such switching of the switches associated with the main coil assembly and the B0 shim coil assembly can be effected in series. However, when the main coil assembly is in the persisted mode, switching of the B0 shim coil assembly must be effected independently of the main coil assembly to provide the required fine adjustment of the central magnetic field.
In addition to the ability to set the central magnetic field to a precise desired value, it is important in many applications that this set value remains stable with time. For example, in NMR spectroscopy, experiments conducted within the apparatus can last over several days and even small variations in the value of the central magnetic field, for example of the order of a few parts per billion, can lead to systematic differences in the spectral results obtained as a result of such experiments. There are essentially two ways in which the central magnetic field value can exhibit time-dependence, that is either as a result of a change in the ambient magnetic field due to external sources, or as a result of variation in the magnetic field generated by the main coil assembly itself.
WO 89/09475 discloses a superconducting electromagnet apparatus which makes use of an assembly of auxiliary coils which magnetically couple to the main coil assembly in order to reduce the effect of variation of the central magnetic field value due to changes in the ambient magnetic field. In such apparatus an assembly of superconducting shielding coils connected in series within a loop is arranged so that the effective areas and mutual and self inductances of the main coils and the shielding coils are such that any change in the ambient magnetic field causes changes in the currents of the main coils and the shielding coils in such a manner as to generate a change in the central magnetic field which opposes the change in the central magnetic field due to the change in the ambient magnetic field alone. The main coil assembly would itself usually partially shield the homogeneity volume from the effect of such changes in the ambient magnetic field even without the use of such shielding coils, but the shielding effect can be significantly increased by the use of the shielding coils.
Furthermore EP 0468425A discloses an active-shield superconducting electromagnet apparatus comprising a first superconducting coil assembly for generating a first magnetic field, and a second superconducting coil assembly for generating a second magnetic field, the second coil assembly being electrically connected in series with the first coil assembly and the two assemblies being arranged such that a resultant, uniform magnetic field is generated in the working volume and the second magnetic field opposes the first magnetic field externally of the apparatus so that the stray magnetic field outside the coil assemblies is very small. This enables personnel to work safely relatively close to the apparatus without requiring an excessive amount of cumbersome and expensive iron shielding.
However the automatic shielding arrangement of WO 89/09475 is no longer effective in relation to conventional active-shield superconducting electromagnet apparatus. Accordingly EP 0468425A proposes an arrangement in which screening coils connected within a closed loop are provided having a number of turns at least an order of magnitude less than the number of turns in the first and second coil assemblies so as to reduce the effect of disturbing influences on the central magnetic field whilst having an insignificant effect on the homogeneity of the central magnetic field. In this case the screening coils are wound from superconducting wire having a critical current such that they revert to the normal conducting state during quenching of the first and second coil assemblies. In this way the maximum contribution which the screening coils can make to the stray magnetic field is rendered insignificant. Furthermore the number of turns of the screening coils is so small that it is a straightforward matter to generate current in the screening coils to provide adequate screening capacity without risk of generating significant stray magnetic field.
With regard to the second effect producing variation of the central magnetic field value over time, that is variation of the magnetic field generated by the main coil assembly itself as a result of variation of the current supplied to the coil assembly, this effect can be caused by the properties of the superconducting wire which is used to wind the coils and which can result in a very slow decrease in the current (typically several parts per billion per hour of the operating current value) in a phenomenon known as xe2x80x9cdriftxe2x80x9d. It is possible to model such drift in terms of an effective residual resistance of the main coils, and to compensate for the drift on the basis of this relationship. The compensating of drift in this way is referred to as xe2x80x9clockingxe2x80x9d. However, in order for such compensation to be effective, it is important that the drift of the current in the compensating coils is insignificant by comparison with the drift of the current in the main coils. Since the effective residual resistance increases significantly as the operating current in the coils becomes comparable with the critical current value of the wire used for winding the coils, this requires the critical current value for a given maximum current in the compensating coils to be greater than a particular minimum value.
Furthermore the conventional B0 shim coil assembly as described above actually has the effect of increasing the rate of change of the central field due to the drift of the current in the main coil assembly.
It is an object of the invention to provide superconducting electromagnet apparatus with a B0 shim coil assembly which, as well as providing fine adjustment of the central magnetic field value, also compensates for changes in the central magnetic field value with time either as a result of changes in the ambient magnetic field or as a result of changes in the field generated by the main coil assembly itself, or as a result of both such effects.
According to the present invention there is provided superconducting electromagnet apparatus comprising a main coil assembly (1; 1xe2x80x2) for producing a central magnetic field in a working volume, main current supply means (5) connected to the main coil assembly for energising and de-energising the main coil assembly, and for persisting the superconducting current flow in the main coil assembly when a desired constant current level has been reached, in order to generate a central magnetic field of high homogeneity in the working volume, a B0 shim coil assembly (2; 2xe2x80x2) for providing fine adjustment of the central magnetic field, the B0 shim coil assembly comprising superconducting shim coil means connected within a closed loop and arranged to magnetically couple with the main coil assembly (1; 1xe2x80x2), auxiliary current supply means (6) connected to the B0 shim coil assembly for supplying current to the closed loop, and for persisting the superconducting current flow in the closed loop when a desired constant current level has been reached, in order to provide fine adjustment of the central magnetic field within the working volume without significantly degrading the homogeneity of the central magnetic field, and control means (31, 38) for controlling the main and auxiliary current supply means (5, 6), wherein the main coil assembly (1; 1xe2x80x2), the B0 shim coil assembly (2; 2xe2x80x2) and the control means (31, 38) are adapted to provide significant compensation for the effect of variation of the magnetic field within the working volume with time whilst maintaining the superconducting current flows in the main coil assembly (1; 1xe2x80x2) and the B0 shim coil assembly (2; 2xe2x80x2).
It should be understood that the term xe2x80x9csignificant compensationxe2x80x9d is used in this context to denote a level of compensation which is such as to lead to a significant improvement in system performance. This would include situations where an experiment or application is achievable with such xe2x80x9csignificant compensationxe2x80x9d, whereas such an experiment or application could not be performed without such compensation.
It will be appreciated that the invention provides an arrangement by which a single closed loop coil assembly can perform the function of a B0 shim whilst at the same time compensating for the effect of variation of the magnetic field within the working volume with time. The coil assembly may be adapted to compensate for the effect of time variation of the magnetic field as a result of time variation of the ambient magnetic field, or alternatively may be adapted to compensate for the effect of time variation of the magnetic field as a result of drift of the current in the main coil assembly. As a further alternative the shim coil assembly may be adapted to compensate for both of these time-varying effects. However in all cases the single closed loop coil assembly serves several functions, and thus avoids the need to provide individual closed loop coil assemblies for performing these functions separately which would result in functional difficulties in view of the fact that each such auxiliary coil assembly would couple inductively with the other auxiliary coil assemblies as well as with the main coil assembly. Furthermore the provision of separate auxiliary coil assemblies for performing the different functions individually would result in additional complications in the design and construction of the apparatus, as well as rendering the apparatus more expensive than apparatus in accordance with the invention in which a single closed loop coil assembly is adapted to perform more than one function simultaneously, that is the above-described shielding and/or locking functions in addition to the fine adjustment of the central magnetic field value.